Many sections of the earth which are contaminated with hazardous materials lie in layers, often 10 or more feet below the surface and are some are 10 to 50 feet thick. The contaminants are often volatiles such as gasoline, and semi-volatiles, such as jet fuel. It is known that such fuels and other such contaminants pose known or possible health hazards. Conventional remediation means, such as incineration or low temperature volatilization in an on-site retort, require excavation, which becomes prohibitively expensive if layers of uncontaminated overburden must be removed. In addition, the costs of transport and incineration itself, as well as site restoration, are quite high.
One proven in situ method of recovering hydrocarbons and similar thermally responsive constituents is to heat the contaminated soil to increase the vapor pressure and to remove such vapors by means of a drying atmosphere such as a steam sweep. Such methods are currently being tested wherein the necessary short-wave-band frequency apparatus heats from the surface down. If semi-volatiles and high boilers are to be removed, a steam sweep at a high temperature is preferable, and this can be realized by evaporating the moisture in the soil to provide an autogenous steam sweep, or by further heating to further increase the vapor pressure, thereby assuring almost complete recovery of the contaminants. However, the efficiency of such existing technology is reduced by the necessity of heating uncontaminated layers of overburden.
Petroleum-rich formations exist from which constituents can be effectively recovered if selectively heated, especially to temperatures at which the free water and much of the water-of-crystallization can be removed. Systems have been successfully developed and tested which employ rows of electrodes emplaced from mined galleries which, when excited by electromagnetic energy, uniformly heat such formations. Radiating antenna-like structures also have been proposed which might selectively heat such deposits. However, such antenna-like heating approaches have encountered problems, and to date few if any successful tests employing antenna-like structures have been reported.
Electromagnetic or radio frequency (RF) heating of earth media or reservoirs containing hydrocarbons or noxious volatile wastes has been the subject of some investigation over the last 10 to 20 years. The objective has been to heat the deposit to assist in the removal of valuable minerals such as oil, or noxious materials such as solvents and liquid fuels. In situ electromagnetic heating technology which has been disclosed to date falls into two major categories: A) bound-wave heating (either low or high frequency), and B) radiated wave heating (high frequency only).
Bound-wave heating structures are those in which the wave is largely contained within a specified volume and is not permitted to radiate significant amounts of energy. The original purpose of radiated wave structures (antenna), on the other hand, was to radiate waves into a lossless dielectric, such as air. Examples of the bound-wave approach appear in U.S. Reissue Pat. No. 30,738, and in U.S. Pat. Nos. 4,140,180, 4,144,935 4,499,585, 4,498,535 and 4,670,634. The successful application of the bound-wave process using the high frequency version is discussed in "Development of the IIT Research Institute RF Heating Process for In Situ Shale/Tar Fuel Extraction--An Overview", presented at the Fourteenth Oil Shale Symposium, Colorado School of Mines, Golden, Colorado, April 1981 by R. D. Carlson, et al. The successful use of a high frequency version of the bound-wave heating to decontaminate hazardous waste spills appears in "Radio Frequency Enhanced In Situ Decontamination of Soils Contaminated with Halogenated Hydrocarbons", presented in the proceedings of the Twelfth Annual Research Symposium, U.S. EPA, April 21-22, 1986, U.S. EPA Publication No. EPA/600/9-86/022 by H. Dev.
Direct application of radiated wave technology to heating lossy media such as soil has not achieved the same degree of success as bound-wave methods. Examples of direct application of antenna technology, intended for radiation in lossless media such as air, to heating lossy media appear in U.S. Pat. Nos. 4,301,865, 4,140,179, 4,457,365, 4,135,579, 4,196,329, 4,487,257, 4,508,168, 4,513,815, 4,408,754, 4,638,863, 2,757,738, 4,228,851, 3,170,519, and 4,705,108.
The lack of reported success in using the radiated wave approach in conductive earth media (as opposed to air) may be attributed to several possibilities. One possibility is that far field radiated wave technology, which was originally developed for radiation into lossless media such as air, has been incorrectly adapted for media which are highly conducting by comparison. Another possibility originates in the misconception that hydrocarbon material can be selectively heated to high temperatures, regardless of the soil matrix, even though such material is both finely divided and widely dispersed in the matrix. Such a misconception may have led to impractical equipment and negative results. An example of a radiating antenna structure designed to recover hydrocarbons (either contaminants or fuels) embedded in loss earth is described in U.S. Pat. No. 5,065,819.
A primary difficulty in directly applying the radiating antenna concept to lossy media is the nature of the very intense fields near the antenna. Such intense fields are of little concern in conventional radiating antennas operated in air, unless extremely high powered pulses are applied. In earth, such intense fields create hot spots which eventually lead to thermal breakdown of the soil. Published literature such as "Electromagnetic Field of an Insulated Antenna in a Conducting or Dielectric Media" by R. W. P. King, et al., IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-31, No. 7, Jul. 1983, as well as results of computer programs, demonstrate that very intense and highly localized fields appear near the ends of dipole antennas and at the power input points (feed points), especially near sharp corners typically associated with such antenna designs. A consequence of disregarding the impact of such intense local fields at the feed point is that the earth medium may swell or melt, making it impossible to withdraw the fairly expensive antenna equipment, as has been known to occur in oil shale deposits. Examples of proposed technology to combat this particular problem, appear in U.S. Pat. Nos. 4,553,592, 4,576,231, and 4,660,636. In contrast, the heating near the electrodes in bound-wave heating of oil shale deposits is far more uniform, and causes much less swelling of the earth so that the electrodes may be easily extracted.
A further example of the use of the bound-wave approach to decontaminate hazardous waste spills is described in U.S. Pat. No. 4,670,634. An example of a radiating antenna structure embedded in lossy earth designed to recover hydrocarbons (either contaminants or fuels) is described in U.S. Pat. No. 5,065,819.
It has been contemplated in the prior art that more extensive radiative heating of the soil can be achieved using the radiating wave technique on account of the differences in dielectric properties of moist and dry soil. As the embedded antenna heats the soil in its immediate vicinity, moisture is boiled out, creating a steam sweep for use in flushing contaminants or hydrocarbons. This leaves the soil in the vicinity dry. The dielectric properties of such dry soil more closely approximates that of a lossless medium like air, and thus wave energy can be radiated through the dry soil to a greater distance with less attenuation than would be the case for moist soil. A significant problem, however, lies in achieving a uniform heating pattern for this purpose. Hot spots develop at certain locations around the exciters, while other locations remain moist. There is a need in the field to develop an apparatus which heats more uniformly and which mitigates the effects of these hot spots.
Yet another difficulty in directly applying the radiating antenna concept to lossy media is in determining how many exciter elements to use, and how closely to space them.
The development of an apparatus for selectively heating a subsurface layer, which does not need to be emplaced from mined galleries, would offer a more cost-effective removal of contaminants in layers substantially beneath the surface of the earth. It would also permit more cost-effective recovery of petroleum deposits which exist beneath an overburden and which require heating to over 100.degree. C.
What is further needed is an apparatus having means for suppressing intense fields and excessive heating near the edges and power input points of the exciters.
There is a further need for an apparatus which can selectively heat a thin subsurface layer, without excessive loss of energy into uninteresting layers above and below.
Finally, there is need for an apparatus in which the exciters are spaced appropriately closely to achieve uniform heating throughout the formation.